Long-distance transmission is mainly used in a scenario such as a sensor network or industrial control. An expected transmission radius of long-distance transmission may reach 2 km or longer. A method for implementing long-distance transmission is narrowband transmission. To be specific, energy is concentrated on narrower bandwidth, to implement longer-distance transmission.
Minimum transmission bandwidth of conventional Wireless Fidelity (WiFi) is 20 MHz. To extend a transmission range, transmission bandwidth may be reduced, for example, transmission is performed on bandwidth of 2 MHz. To improve transmission efficiency and enable WiFi to better work in a scenario in which devices are highly densely distributed, in 802.11ax, orthogonal frequency division multiple access (OFDMA) and uplink multi-user multiple-input multiple-output (UL MU-MIMO) are introduced. OFDMA includes UL OFDMA and downlink orthogonal frequency division multiple access (Downlink OFDMA, DL OFDMA). UL OFDMA and UL MU-MIMO may be collectively referred to as UL MU transmission, and DL OFDMA and DL MU-MIMO (introduced in the 802.11ac standard) may be collectively referred to as DL MU transmission. In long-distance narrowband transmission, the DL MU transmission is usually difficult to implement because of a limitation of maximum transmit power of an access point (Access Point, AP). When the AP performs the DL MU transmission, power obtained by each user is less than that in single user (SU) transmission, and therefore, a purpose of long-distance transmission cannot be achieved. In other words, a DL signal may be incapable of reaching a station (STA). However, the UL MU transmission is still feasible. Energy of the UL MU transmission is jointly provided by a plurality of users, and transmit power of each user is limited only by a capability of the user and maximum transmit power that is of a device and that is required in a country and a region. Therefore, the UL MU transmission does not affect receive power that is of a transmit signal of each user and that is at the AP. In long-distance narrowband UL MU transmission, because a sub-channel occupied by each user is relatively narrow, channel frequency selectivity is apparent. In other words, channel characteristics of different sub-channels vary greatly. In this case, a UL channel needs to be measured, to allocate an appropriate sub-channel to each STA and determine an appropriate modulation and coding scheme (MCS), so that the UL MU transmission is efficiently performed.
A UL channel measurement method in the prior art is shown in FIG. 1. An access point (AP) sends a TF-S (Trigger for Sounding) frame to indicate a list of STAs that need to perform a measurement process and a set of sub-channels that need to be measured. In addition, each STA simultaneously sends channel sounding messages only on sub-channels that are considered to be idle by the STA and that are in the set of sub-channels indicated by the AP, so that the AP measures a UL channel. However, in a long-distance transmission case, each STA simultaneously sends the sounding messages on a plurality of sub-channels that are considered to be idle by the STA, and therefore, energy is distributed on the plurality of sub-channels, and in this case, the sounding messages may be incapable of reaching the AP. In addition, when the STAs send the sounding messages to the AP, only one STA performs sending in each timeslot, and before obtaining a measurement result of the UL channel, the AP needs to sequentially receive the sounding messages sent by all the STAs in the list of STAs. Therefore, time overheads of a UL channel measurement execution process are relatively high.